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6.4 Halftoning
6.4.3 Spot function screens
Spot functions describe the dot shape using a mathematical function which, given the position of a pixel within a halftone cell, produces a number which says where in the sequence of
pixels that pixel should be marked. This technique is very flexible, because it allows a single
function to define a halftone cell at any size or angle.
In the Harlequin RIP the common dot shapes are provided by spot function screens. This
means that all the screening options in the Edit Style dialog box are available, including the
benefits of using Harlequin Precision Screening (HPS).
6.4.4 Threshold screens
As an alternative to a spot function screen, a threshold screen describes a halftone to the Harlequin RIP by providing the order in which to mark pixels explicitly in a table. The table has
as many entries as there are pixels in the halftone cell.
A consequence of this is that a threshold screen is always defined in terms of a fixed cell size
in pixels, whereas a spot function can be applied to arbitrary sizes of screen dot. This means
that the screen cell for a spot function will occupy more pixels, but the same physical area on
the output, if either the resolution is increased or the frequency (number of halftone cells per
device independent unit distance) decreased, but a threshold screen will get physically
smaller and its frequency increase if the resolution is increased.
A further consequence is that HPS and other techniques which vary the actual pixels of a
screen cell cannot be applied to threshold screens. On the other hand, threshold screens can
be controlled very precisely, pixel for pixel, and are therefore suitable for describing Frequency Modulated (FM) screens if the cell is large enough. While threshold screens can be
used to describe conventional “clumped” dots, they are less flexible because of their resolution dependence and absence of rotation to a specified angle (at least in their simplest form).
Neither kind of screen is suitable for describing non-tessellating screening techniques, such
as error diffusion.
The PostScript language offers several variations on threshold screens, which allow greater
or lesser control over the sizes of the tables involved and the tessellation of the tiles (equivalent to the “angle”) in a spot function.
There are no threshold screens in the standard set supplied with the RIP but it is possible to
use threshold screens embedded in a job, or installed in the RIP by your supplier.
6.4.4.1 Using threshold screens
In the Edit Style dialog box, when you have selected a threshold screen in the Dot shape list
and Override dot shape in job is selected, the following options are not available:
•
Frequency
•
Override frequency in job
•
Angles (for all process and spot colors)
•
Override angles in job
Harlequin PLUS Server RIP v9.0: June 2011
OEM Manual
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